Free Chlorine vs Combined Chlorine: What Your Test Is Telling You
By The Aqualitx team at STEMania · Updated
Most test kits report two or three chlorine numbers, and they mean very different things. Reading them correctly is the difference between a sanitized pool and one that just smells like chlorine.
The three numbers
- Free chlorine (FC) is the chlorine still available to kill bacteria and algae. This is the number that keeps the pool safe.
- Combined chlorine (CC) is chlorine that has already reacted with sweat, urine and other nitrogen compounds, forming chloramines. It's spent — and chloramines cause the "chlorine smell" and red eyes people blame on too much chlorine.
- Total chlorine (TC) is simply the two added together: TC = FC + CC.
Many kits measure FC and TC directly, so you get CC by subtracting. If FC and TC read the same, combined chlorine is zero — good.
What free chlorine should be
In an unstabilized pool, 1 to 4 ppm of free chlorine is the classic range. But most outdoor pools use stabilizer (cyanuric acid, or CYA), which shields chlorine from the sun — and also holds most of it in reserve. The more CYA in the water, the more free chlorine you need for the same killing power.
A reliable minimum is about 7.5% of your CYA level: at 40 ppm CYA, keep free chlorine at 3 ppm or more; at 60 ppm, at least 4.5. That's why a pool can test at 2 ppm and still grow algae — the number looked fine, but the CYA made it too weak. The chlorine calculator sizes doses to your CYA.
When combined chlorine means it's time to shock
The long-standing line is 0.5 ppm. Below it, CC is usually just test noise or normal background. Above it, chloramines are building faster than your free chlorine clears them.
Clearing them takes breakpoint chlorination: raising free chlorine to about 10 times the combined chlorine reading. At 0.8 ppm CC, that's 8 ppm free chlorine. Reaching breakpoint oxidizes the chloramines; falling short can make them worse for a while. The shock calculator works out the dose for your pool.
Is your CC reading even real?
Combined chlorine is a small number measured by counting drops, so the test's resolution matters. On a 10 mL sample, each drop is worth 0.5 ppm — so one stray drop reads as "0.5 CC" and looks like a shock is due. A 25 mL sample reads in 0.2 ppm steps and gives a much more trustworthy answer. Re-test before committing to a big shock.
Combined chlorine that won't go away
If CC shows up on test after test, even at low levels, free chlorine isn't keeping ahead of demand. Before reaching for a shock, try holding free chlorine at its target every day for a couple of weeks instead of letting it sag between tests. That alone clears most low-level chloramines. If it's still there after that, a breakpoint shock is the next step.
The short version
- Free chlorine is the number that sanitizes. Combined chlorine is spent chlorine.
- Keep free chlorine at or above 7.5% of your CYA.
- Combined chlorine above 0.5 ppm: shock to about 10× the CC reading.
- Use a 25 mL sample before trusting a small CC number.